1. Field of the Invention
The present invention relates to an ink jet printing apparatus that imparts to a print medium a liquid compound in addition to inks to produce a color image with excellent color reproduction and excellent uniformity of color. The present invention also relates to an image processing method for the ink jet printing apparatus and a control program for executing the image processing method.
2. Description of the Related Art
An ink jet printing apparatus ejects ink droplets onto a print medium such as print paper to form an image. With an ink jet printing apparatus of a type that uses an electrothermal transducer as an ejection energy supply means and imparts thermal energy to ink to generate a bubble in the ink and eject an ink droplet by a bubble-generated pressure, as disclosed in Japanese Patent Application Publication Nos. 61-59911 (1986), 61-59912 (1986) and 61-59914 (1986), it is possible to realize a high-density multi-orifice print head easily which can produce a high-quality, high-resolution image at high speed.
Because of its advantages, such as relative ease with which to form a color image, and to form an ink jet printing apparatus by a relatively low cost and a small size, the ink jet printing apparatus is rapidly gaining popularity.
With a widespread use of the ink jet printing apparatus in recent years, there is a growing demand on the ink jet printing apparatus for as high an image quality as a silver salt picture. This in turn has resulted in growing technical demands for an enhanced color density, a wider color reproduction range and a color uniformity of an ink jet-printed image.
Under these circumstances, a variety of proposals have been made so far to realize a stabilized ink jet printing and an improved quality of images printed by the ink jet printing. One such proposal for a print medium involves applying a filler material and a size agent to the surface of base paper of a print medium. For example, a technique is disclosed which coats the base paper with porous microparticles as a filler material that adsorbs a colorant, to form an ink receiving layer of the porous microparticles. As print media using this technology, ink jet coated paper is on the market.
In addition to special processing performed on a print medium, various kinds of treatments have also been proposed for inks applied to the print medium to secure a stable, high-quality printed result. Some representative proposals will be described in the following.
A first such proposal is to incorporate in ink a volatile solvent and a penetrating solvent. For example, as described in Japanese Patent Application Laid-open No. 55-65269 (1980), a chemical agent such as surfactant that enhances a penetrating performance is incorporated in ink to quicken the fixing of the ink onto a print medium. Further, Japanese Patent Application Laid-open No. 55-66976 (1980) discloses a method which uses an ink made up mainly of a volatile solvent to quicken the fixing of ink.
A second proposed method involves applying a liquid compound called a processing liquid in addition to the ink onto a print medium so that the ink and the liquid compound mix and react with each other on the print medium. The liquid compound, when it reacts with the ink, produces a variety of effects, such as improving an image density and a water resistance and suppressing bleeding. Either before or after the ink is applied to the print medium, the liquid compound is ejected onto ink landing positions to mix and react with the ink on the print medium, producing the above-mentioned effects to form a good printed image.
Japanese Patent Application Laid-open No. 63-60783 (1988), for example, discloses a method which applies a liquid compound containing a basic polymer to a print medium and then ejects ink containing anionic dye to form an image. Japanese Patent Application Laid-open No. 63-22681 (1988) describes a printing method which mixes on a print medium a first liquid compound containing a reactive chemical species and a second liquid compound containing a chemical agent that reacts with the reactive chemical species. Further, Japanese Patent Application Laid-open No. 63-299971 (1988) describes a method which first applies to a print medium a liquid compound containing an organic chemical compound having two or more cationic groups for each molecule and then ejects inks containing an anionic dye to form an image. Further, Japanese Patent Application Laid-open No. 64-9279 (1989) describes a method that applies to a print medium an acidic liquid compound containing succinic acid and the like followed by inks containing anionic dye to form an image.
Japanese Patent Application Laid-open No. 64-63185 (1989) discloses a method that applies a liquid compound, which renders a dye insoluble, to paper before ejecting ink droplets. Japanese Patent Application Laid-open No. 8-224955 (1996) discloses a method that uses a liquid compound containing cationic substances with different molecular weight distribution areas, along with inks containing an anionic chemical compound. Japanese Patent Application Laid-open No. 8-72393 (1996) describes a method that uses a liquid compound containing a cationic substance and powdered cellulose along with inks. These methods produce a high image density, a good printed quality and a good water resistance. It is also described in these publications that printed images thus produced have good characteristics in terms of color reproduction and bleeding. Further, Japanese Patent Application Laid-Open No. 55-150396 (1981) proposes forming an image on a print medium with a dye ink and then applying an insolubilizer that reacts with the dye to form a lake.
A third method involves mixing inks with a microparticle-containing liquid compound on a print medium.
Japanese Patent Application Laid-open No. 4-259590 (1992) discloses a method which first applies to a print medium a colorless liquid containing colorless inorganic microparticles and then ejects non-aqueous recording liquid. Japanese Patent Application Laid-open No. 6-92010 (1994) discloses a method that first applies to a print medium a solution containing microparticles or a solution containing microparticles and binder polymer and then ejects inks containing a pigment, a water-soluble resin, a water-soluble solvent and water. These methods are described to be able to produce images with good printed quality and color characteristics irrespective of a kind of paper. The method of applying a processing liquid is effectively used also for print media that are not given a special treatment on the surface, such as plain paper, in that it can enhance the image quality.
In the method of applying a processing liquid as well as inks to a print medium, it is necessary to generate an ejection signal for the processing liquid in addition to an ink ejection signal. Generally, a host computer sends image data to a printing apparatus where it is processed to generate only ejection information for each ink. Thus, the processing liquid ejection signal is separately generated by performing logic operations on the ink ejection information in the printing apparatus, as disclosed in Japanese Patent Application Laid-open Nos. 08-216386 (1996) and 08-281974 (1996).
For example, image data that is received in the form of 8-bit data (256 gray levels) for each of RGB colors is converted by a three-dimensional lookup table (LUT) into CMY data. The CMY data is further subjected to gamma processing to be transformed into 8-bit data for each of CMYK colors. The CMYK 8-bit data is further converted by binarization processing into binary data, or ejection data for each of CMYK ink colors. Based on the binary ejection data, the processing liquid ejection data S is determined from logic operations.
After having investigated a variety of ink jet printing techniques including those described above, the inventors of this invention have found that although these techniques produce satisfactory results in terms of the respective technical problems, they often degrade other ink jet printing characteristics.
For example, a print medium that is coated on its base paper surface with a filler and sizing agent (simply referred to as coated paper) is recognized as a technology that enables a formation of high quality images.
Generally, to produce an image with high color saturation, it is known that a colorant must remain on the surface of a print medium in a single-molecular state without being aggregated. Porous microparticles in the coated paper have a function of fixing a colorant in the single-molecular state. However, to obtain a high image density and high color saturation, it is essential to form a thick ink reception layer made up of a large amount of porous microparticles which covers the base paper to hide it from the colorant in the ink applied to the print medium. This gives rise to a problem that the texture of the base paper is lost. One of possible reasons that an ink reception layer so thick as will lose the texture of the base paper needs to be formed is that the colorant may not adsorb to the porous microparticles efficiently.
In the explanation that follows, coated paper with one ink reception layer is contemplated. FIG. 5 is a cross section schematically showing a surface of the coated paper and its surrounding. In the figure, denoted 901 is base paper and 903 an ink reception layer. The ink reception layer 903 has porous microparticles 905 and an adhesive 907 that binds them together. When an ink is applied to the coated paper, the ink penetrates into voids between the porous microparticles 905 by capillary attraction to form ink penetration portions 909. As shown in the figure, since the porous microparticles in the ink reception layer varies in density depending on locations, the way the ink penetrates by the capillary attraction differs from one location to another. Thus, during the process of ink penetration, the colorant cannot contact the surfaces of the porous microparticles uniformly, failing to adsorb to the porous microparticles efficiently.
Further, there are portions in which the adhesive 907 impedes ink penetration, preventing the ink from penetrating into the ink reception layer 903. These portions thus cannot contribute to an improvement of color saturation. From the reasons described above, the conventional coated paper cannot adsorb a colorant in a single-molecular state as efficiently as it should, considering the volume of porous microparticles. As a result, to obtain a high quality image a large volume of porous microparticles is needed, which in turn impairs the texture of the base paper.
Further, although the technique cited as the first method, which mixes a volatile solvent in inks, can improve the fixing of the inks to a print medium, there have been cases where an image density deteriorates or a color reproduction range, considered important in the printing on plain paper or in the color image printing, decreases. Further, with the technique cited as the second method, which applies a liquid compound to inks on a print medium, since the colorant in the inks can be left on the surface of the print medium, a print with a high image density can be obtained. However, there are cases where the color reproduction range and color saturation level obtained are not satisfactory probably due to the colorant aggregating on the surface of the print medium. Further, with the technique cited as the third method, which mixes inks with a microparticle-containing liquid compound on a print medium, although the condition of the surface of the print medium can be improved by the application of the microparticle-containing solution, it is not possible to produce so vivid an image as can be realized with coated paper. As for non-aqueous inks, there are limitations imposed on the selection of colorants and the way the colorants are applied.
In the embodiment of the present invention described in the following, a liquid compound will be shown which can eliminate the above-described problems experienced with the conventional liquid compounds. However, the following problems still exist with the application of liquid compounds including the conventional liquid compounds.
In the method that applies a liquid compound (also referred to as a “processing liquid”) as well as inks, there are limitations on the amount of inks and processing liquid that can be applied to a print medium. This narrows a degree of freedom of image reproduction. Generally, ink is composed of about 95% solvent by weight. That is, a colorant, such as dye and pigment, occupies only 5% of droplets ejected. To send the ejected droplets flying stably, a large volume of solvent as a carrier needs to be ejected at the same time and, after the ink droplets land on the print medium, the carrier is required to be fixed quickly. However, since there is a limit on the amount of liquid that the print medium can absorb, if an ink volume in excess of the ink accommodation capacity of the print medium is applied, the ink will flow or bleed. For this reason, there is a limit on the volume of ink that can be applied to the print medium and this constitutes an obstacle in the way of enhancing an image quality. Where a processing liquid is applied, a solvent for the processing liquid needs also to be applied in addition to the solvent for the inks and therefore the applicable volume of inks is more limited than when only the inks and their solvent are used. The limitation on the applicable volume also leads to a problem of reduced degree of freedom in the image processing for generating ejection data, i.e., a problem that the image processing must consider the ink and processing liquid application volumes.
Ejection data of a processing liquid has conventionally been generated by performing the color conversion processing on received RGB image data to form ejection data for each ink color and performing logic operations on the ink ejection data, as described above. During this process, considering the limitation on the application volumes in generating the processing liquid ejection data will make the calculation volume prohibitively huge.
That is, in a pseudo-gray scale representation based on ejection/non-ejection of ink dots, a color or density is represented not for each pixel, a unit for data processing, but for a unit area of a certain expanse made up of a plurality of pixels. Thus, a frequency of ejection of each ink is also determined for each unit area and, in generating the ink and processing liquid ejection data that considers the application volume limitations, it is necessary to take into account the ink ejection data in this entire unit area. This in turn requires a relatively large volume of processing calculations to generate the processing liquid ejection data.
With an improvement of image quality in recent years, a variety of printing methods are coming into use making the printing operation complicated. They include, for example, a dark-light ink printing using low-density photo inks (PC, PM) in addition to ordinary CMYK inks, an ejection volume modulation printing that changes an ink ejection volume to improve a granularity, and a combination of these. A dark ink and a light ink have different amounts of colorant in a single droplet. Hence, with these printing methods, the required processing liquid application volume varies depending on the kind of the printed dot. In this condition, these known methods for generating the processing liquid ejection data are required to perform different processing for each print data with a different ink kind and a different ejection volume, making the system complex and increasing the manufacturing cost.
The present invention has been accomplished to overcome the problems experienced with the conventional methods. This invention provides an image processing method which, even in an ink jet printing method that applies a liquid compound for improving the fixing of inks to a print medium in addition to the inks, can generate liquid compound ejection data considering limitations on ink application volumes, without increasing a processing load. This invention also provides an ink jet printing apparatus using the image processing method, and also a control program that executes the same method.